Friction drive is a very good way to drive a astronomical telescope. This drive has obvious advantages to an astronomical telescope: the structure design and manufacturing is simple. For the same performance requirement, friction drive has the lowest cost, which means highest performance to price ratio; it has no cyclic accumulated error and no no-load return; the telescope drive is stable without vibration, and the response is quick, able to obtain very high drive precision at low speed; the friction driving wheel and driven parts can have high drive ratio, which means reduced motor torque and low-speed performance requirements, therefore, serial commercial motors can be used, normally without the need to have a tailor-made motor, thus greatly reducing the cost.
Presently, an external cylindrical rolling friction drive is adopted for many known large-caliber telescopes at home and overseas. The drive pair consists of a driving wheel and driven wheel, and the driven wheel is directly mounted on the same shaft of the telescope body. The driving wheel presses on the driven wheel with the action of the pressing device, the motor drives the driving wheel, and the movement is transferred by the friction force between the external cylindrical contact surfaces of the driving and driven wheels. In rolling friction drive, the rotating axial lines of the driving and driven wheels should be parallel as ideal positions in space, however, due to mounting error and deformation of supporting structure operation long-term operation under gravity, there is an angle between their rotating axial lines in space. The angle between the driving wheel rotating axial line and driven wheel rotating axial line at their junction within the tangential plane is referred to as torsion angle, as shown in FIG. 1. Torsion angle is a special non-linear interference, which results in the change of the contact surfaces of driving wheel A and driven wheel B, greatly reducing the contact area from a rectangular contact zone over the whole thickness to a very small part of circular section of the original contact surface. This will lead to sliding displacement between the driving wheel and driven wheel.
The friction drive with the presence of a torsion angle is similar to torsion wheel friction drive. Because of the presence of the torsion angle, the friction force produces a component in the direction of the friction wheel rotating axial line, under the action of this component, the friction wheel will move in the axial direction, however, such axial movement is restricted, and elastic deformation of structure will take place instead. In the working process of the astronomical telescope with friction drive, this elastic deformation gradually increases, so does the corresponding elastic recovering force. When the elastic recovering force has exceeded the friction force between the friction wheel contact surfaces, the friction wheels will “skip suddenly” back to the balanced position. Normally, the driven wheel is connected with the telescope proper, with great weight and very high rigidity, and comparatively, the supporting rigidity of the driving wheel is not so high. Therefore, such “sudden skip” mostly occurs on the driving wheel. The interference produced by the torsion angle on the telescope is a special non-linear interference, finally, it will result in fluctuation of the driven load, and affecting the low speed stability of the telescope and its precision to track the celestial bodies. Presently in the area of astronomical telescope, there is no practically feasible technical solution to this torsion angle non-linear interference. Only the robustness of the control algorithms (such as the classical PID) of the telescope direction tracking system is depended to suppress the sudden skip caused by torsion angle. However the effect is usually not satisfactory and it can only alleviate to a certain extent the effect of sudden skip on the telescope low speed tracking precision.
The patent application with the filing No. CN200810020722.8 disclosed a dynamic correcting system for the rolling friction drive rotating shaft torsion angle of extremely large astronomical telescopes. This dynamic correcting system can always maintain tangentially parallel the driving wheel rotating axial line and driven wheel rotating axial line in the friction drive process, to ensure synchronous movement of driving wheel and driven wheel, avoid the occurrence of “sudden skip” and ensure stable operation of the telescope. The patent application with the No. CN200810020722.8 is mainly based on mechanical design.